Today it is not possible to use the environment variable mechanism to configure a maven mirror and provide credentials for it. confer:
https://github.com/jboss-openshift/cct_module/blob/master/s2i-common/common.sh
It would be possible to add a few environment variables for supporting the configuration of a server with an id matching the mirror, something like:
<servers>
<server>
<id>$mirror_id</id>
<username>$mirror_user</username>
<password>$mirror_pwd</password>
</server>
</servers>

private key with pass phrase could also be supported.

If there is interest for it it is something I could contribute.